The present invention concerns mechanical clockwork movements, in particular in watches, and it concerns certain arrangements for reducing the errors in isochronism caused by imperfections in the regulating system of the watch.
The tourbillon invented two centuries ago by Abraham-Louis Breguet is a device that reduces errors in isochronism resulting from effects produced particularly by gravity on the regulating organs of the watch, because of the inevitable faulty poising of such organs. As the oscillator and the escapement are mounted in a carriage or cage which rotates about an axis parallel to the axis of the sprung balance assembly, the gravity component that is exerted in the perpendicular plane to these axes performs a continuous rotation in relation to the organs, such that each rotation of the cage leads to compensation for the effects of unbalance in that plane and thus improves the working regularity of the watch when worn, especially when the watch is in a vertical position. In order to simplify the terminology, the term “tourbillon” is used here to mean both devices in which the axis of the balance coincides with the rotational axis of the cage (for example according to Breguet or according to CH Patent No. 262 017) and devices often called “carrousels”, where those axes are distinct (see for example CH Patent Nos. 30 754 and 256 590 and EP Patent No. 846 987).
Given that a conventional tourbillon only compensates imperfectly for the effects of gravity, watchmakers seeking to further improve the isochronism of high quality mechanical watches have designed tourbillons with two or three axes of rotation that are all perpendicular to each other, disclosed in particular in the Patent Publication Nos. GB 2 027 232, CH 693 832, EP 1 465 024 and WO 2004/077171. These constructions constitute a remarkable feat, but they occupy a spherical space and thus can only be fitted to an extremely thick watch.
According to WO 03/017009, a similar object is achieved by means of a tourbillon with two axes of rotation which intersect each other at an angle other than 90 degrees, for example 30 degrees. This construction is more compact as regards height than a construction with two perpendicular axes, but it remains considerably thicker than a conventional tourbillon movement.
FR Patent No. 2 784 203 presents yet another method of reinforcing the compensation provided by the tourbillon. The tourbillon, the barrel that drives it and the gear train connecting these two elements are mounted on a rotating plate completing one revolution per hour, whose axis of rotation is parallel to that of the tourbillon. This arrangement in a way forms a tourbillon on a carrousel, with the tourbillon revolving about the centre of the rotating plate.
Another method of improving isochronism was formulated in the 1930s by M. Vuilleumier and was published in CH Patent No. 156 801, consisting in using two normal regulating systems each comprising a sprung balance and an escapement in a single clockwork movement, these two systems being coupled to the going train by means of a differential gear fulfilling the duel function of distributing the drive energy in equal parts to the two systems and averaging the working thereof to regulate the speed of the going train. This principle did not meet with success, probably because the theoretical gains were offset by losses in efficiency in the additional gear trains, in particular in the differential gear. However, with the current tourbillon fashion, various watchmakers have returned to this principle to create watches comprising two or more tourbillons coupled by a differential gear. Such arrangements confer a prestigious aspect on the watches, but it remains to be seen whether they really improve isochronism compared to a watch with a single tourbillon.